1. Field of the Invention
This invention relates generally to the field of data modems. More particularly, this invention relates to a data modem using data compression as a criterion for selection of one of a plurality of constellations for enhanced integrity of data transmission.
2. Background
Data compression has been used by several manufactures of asynchronous data modems to increase the throughput of their modems. It is well known that by proper compression of data to be transmitted, substantial increases in effective transmission speed can be obtained. Unfortunately, not all data can be efficiently compressed. While such devices may often be able to more than double or quadruple the effective data rate, the process is highly data dependent and cannot be absolutely relied upon. Efficient compression is highly dependent upon the inherent redundancy of the data being compressed. Therefore, it is not always possible to depend upon data compression to provide a required increase in transmission throughput.
In U.S. Pat. No. 4,646,061 to Robert Bledsoe, which is assigned to the assignee of the present invention and incorporated herein by reference, a plurality of data compression tables are used to efficiently compress a variety of data types. This particular patent uses variations of Huffman coding to produce efficient compression of a variety of data types by matching the data being compressed to the compression algorithm. Other adaptive schemes are known in the art to allow a greater likelihood of obtaining efficient data compression.
Class 5 of Microcom's.RTM. MNP.TM. error-correction protocol incorporates an apparently adaptive form of data compression which is claimed to provide between 1.3 to 1 and 2 to 1 data compression depending upon the data. Typical compression ratios are said to be approximately 1.6 to 1. This data compression is used in a conventional manner to increase the data throughput of the modem. Class 4 of the MNP protocol incorporates Adaptive Packet Assembly.TM. which adapts the size of packets transmitted so that when larger numbers of errors are occurring, packet size is reduced to reduce wasted bandwidth when retransmissions are required. Other known types of data compression can frequently yield compression of greater than 4 to 1.
One example of an appropriate algorithm for data compression with the present invention is the "Ziv Lempel" algorithm or modification thereof, available by license from BT Datacom, Herndon, Virginia. This algorithm can be implemented in many cases using an eight bit processor with about 16K of memory.
Each of the compression schemes described above, and others known in the art, enhances data throughput by data compression. It is known to use the compression ratio obtained as a feedback mechanism used to select an efficient compression algorithm. However, none of the known data compression art is yet able to reliably provide a relatively constant and dependable data throughput using data compression while increasing the communication reliability and enhancing throughput. In synchronous data transmission, this level of reliability is highly advantageous.
Data throughput may be enhanced in a variety of ways. Consider for example, a 19,200 BPS data compressing modem which is achieving a compression ratio of 1.25 (often expressed as 1.25 to 1 or 1.25:1). One would naturally expect that a data throughput of 1.25 X 19,200=24,000 BPS would be achieved. However, since the transmission bandwidth of the channel is being pushed to the current technology limits in the case such of high speed modems, a substantial number of retransmissions, retrains, errors and other line impairment related disruptions are typical. Thus, with a poor transmission line that is not able to adequately support the 19,200 BPS transmission rate, the effective throughput can be radically reduced by errors, speed changes, retransmissions, retrains and other overhead so that less than the 19,200 BPS rate is actually achieved. Thus, although data compression can clearly enhance the throughput of a data modem in many instances, there is rarely a guarantee.
Rather than gamble on the ability of the data compression algorithm's ability to provide increased transmission throughput, the present invention utilizes whatever compression is available to enhance the reliability of transmission so that fewer retransmissions, retrains and other throughput reducing occurrences are minimized. Thus, in the above example, the user obtains a substantially greater likelihood of achieving the desired 19,200 BPS throughput even in the face of substantial line impairments and unpredictable compression ratios with use of the present invention.
In addition, the present invention is particularly well suited to use in synchronous data modems, but this is not limiting. The prior compression techniques which are aimed only at enhancing throughput are difficult to utilize in a synchronous network. To use them requires extensive buffering and flow control which is not normally present and which are undesirable in such networks. Providing these additional functions can substantially impact the amount of hardware associated with each modem while still providing no guarantee of increased data throughput. The present invention is well suited to such synchronous networks since the end to end throughput is essentially unaltered with the additional bandwidth obtained by compression utilized to facilitate greater immunity to impairments.
In conventional synchronous data modems, a fixed constellation pattern is utilized for a selected bit rate of transmission. An exception is that during initial startup a training sequence containing multiple sets of constellations are sometimes used.
U.S. Pat. No. 4,780,883 to O'Connor et al., which is assigned to the assignee of the present invention and incorporated by reference herein, describes a method and apparatus for effecting speed changes i a data modem. Speed changes are effected based upon the maximum speed which a channel can support without undue errors and are implemented in part by change in the number of constellation points transmitted. Data compression is not disclosed.